CBSE Class 9 Science NCERT Solutions: Chapter 11, Work and Energy

Gurmeet Kaur

May 25, 2018 11:29 IST

Class 9 Science NCERT Solutions

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Now, solving the NCERT questions becomes quite easy with the help of the solutions provided here as these will help you get a better approach to the right explanation for each question.

Not only do these NCERT Solutions for Class 9 Science strengthen a student’s foundation in the subject, but also give them the ability to tackle different types of questions easily. This helps students prepare effectively for the exams and score good marks.

In this article we are providing the NCERT Solutions for CBSE Class 9 Science chapter 11, Work and Energy. All the questions have been solved in the most appropriate and simple way so as to help students access the right study material for the exam preparation.

Main topics discussed in Class 9 Science chapter- Work and Energy are:

Work and its Scientific conception

Work done by a constant force

Energy

Kinetic energy

Potential energy

Law of conservation of energy

Rate of doing work

Commercial unit of energy

Students may download all the NCERT Solutions for CBSE Class 9 Science chapter – Work and Energy, in the form of PDF.

Some of the questions and their solutions from NCERT Solutions for Class 9: Work and Energy, are as follows:

Q.Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Sol.

Expression for the work done when the displacement in the position of the object is along the direction of the force, is

W = Fs

where F = force applied, and s = displacement.

Q.Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.

(a) Seema is swimming in a pond.

(b) A donkey is carrying a load on its back.

(c) A wind mill is lifting water from a well.

(d) A great plant is carrying out photosynthesis.

(e) An engine is pulling a train.

(f) Food grains are getting dried in the sun.

(g) A sail boat is moving due to wind energy.

Sol.

(a) Work is being done by Seema because she displaces the water by applying the force.

(b) No work is being done by the donkey because the direction of force, i.e., the load is vertically downward and displacement is along the horizontal. If displacement and force are perpendicular then no work is done.

(c) Work is done, because windmill is lifting the water, i.e., it is changing the position of water.

(d) No work is done, because there are no force and displacement.

(e) Work is done, because the engine is changing the position of the train.

(f) No work is done because there is no force and no displacement.

(g) Work is done because the force acting on the boat is moving it.

Q.A battery, lights a bulb. Describe the energy changes involved in the process.

Sol.

In the case given in the question, the battery has chemical energy which is converted into energy. Electric energy provided to the bulb further converted into light energy.

Q.The potential energy of a freely falling object decreases progressively. Does this violate flaw of conservation of energy? Why?

Sol.

It is true that the potential energy of freely falling object decreases progressively. But as the object falls down, its speed increases progressively, i.e., the kinetic energy of the object increases progressively (kinetic energy will increase with the increase in speed). Now we can say that the law of conservation of energy is not violated, because the decrease in potential energy results in the increase of kinetic energy.

Q.Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

Sol.

When we push a huge rock, then the rock also exerts a huge force on us (according to Newton's third law of motion). The muscular energy spent by us in the process is used to oppose the huge force acting on us due to the rock.

Q. Write an expression for the work done when a force is acting on an object in the direction of its displacement.

Sol.

Expression for the work done when the displacement in the position of the object is along the direction of the force, is

W = Fs

Where F = force applied,

s = displacement.

Q. Look at the activities listed below. Reason out whether or not work is done in the light of your understanding of the term 'work'.

(a) Seema is swimming in a pond.

(b) A donkey is carrying a load on its back.

(c) A wind mill is lifting water from a well.

(d) A great plant is carrying out photosynthesis.

(e) An engine is pulling a train.

(f) Food grains are getting dried in the sun.

(g) A sail boat is moving due to wind energy.

Sol.

(a) Work is being done by Seema because she displaces the water by applying the force.

(b) No work is being done by the donkey because the direction of force, i.e., the load is vertically downward and displacement is along the horizontal. If displacement and force are perpendicular then no work is done.

(c) Work is done, because windmill is lifting the water, i.e., it is changing the position of water.

(d) No work is done, because there are no force and displacement.

(e) Work is done, because the engine is changing the position of the train.

(f) No work is done because there is no force and no displacement.

(g) Work is done because the force acting on the boat is moving it.

Q. A battery, lights a bulb. Describe the energy changes involved in the process.

Sol.

In the case given in the question, the battery has chemical energy which is converted into energy. Electric energy provided to the bulb further converted into light energy.

Q. The potential energy of a freely falling object decreases progressively. Does this violate flaw of conservation of energy? Why?

Sol.

It is true that the potential energy of freely falling object decreases progressively. But as the object falls down, its speed increases progressively, i.e., the kinetic energy of the object increases progressively (kinetic energy will increase with the increase in speed). Now we can say that the law of conservation of energy is not violated, because the decrease in potential energy results in the increase of kinetic energy.

Q. Does the transfer of energy take place when you push a huge rock with all your might and fail to move it? Where is the energy you spend going?

Sol.

When we push a huge rock, then the rock also exerts a huge force on us (according to Newton's third law of motion). The muscular energy spent by us in the process is used to oppose the huge force acting on us due to the rock.